FIG. 1 is a diagram showing the general structure of an inverter-driven electric chain block. As shown in the figure, the electric chain block has an inverter controller 11, a lifting-lowering motor 12, a speed reducer 13, and a control box 14. The control box 14 has a lifting button 14a and a lowering button 14b, which are two-step pushbutton switches. When the lifting button 14a is pressed to a first step, the control box 14 outputs a low-speed lifting signal to the inverter controller 11. When the lifting button 14a is pressed to a second step, the control box 14 outputs a high-speed lifting signal to the inverter controller 11. When the lowering button 14b is pressed to a first step, the control box 14 outputs a low-speed lowering signal to the inverter controller 11. When the lowering button 14b is pressed to a second step, the control box 14 outputs a high-speed lowering signal to the inverter controller 11.
Upon receiving the low-speed lifting signal, the high-speed lifting signal, the low-speed lowering signal, and the high-speed lowering signal from the control box 14, the inverter controller 11 supplies the lifting-lowering motor 12 with low-speed lifting electric power, high-speed lifting electric power, low-speed lowering electric power, and high-speed lowering electric power, respectively, of predetermined frequency, thereby causing the lifting-lowering motor 12 to rotate forward or reverse at low or high speed. Consequently, a sheave 15 rotates forward or reverse at low or high speed through the speed reducer 13, and a chain 16 engaging the sheave 15 is wound up or unwound at low or high speed. Thus, a load 18 suspended from the lower end of the chain 16 through a hook 17 is lifted or lowered at low or high speed.
In the above-described electric chain block, an electric current (hereinafter referred to as “motor current”) supplied from the inverter controller 11 to the lifting-lowering motor 12 can be divided into an exciting current (an electric current needed to generate a magnetic flux) and a torque current (an electric current proportional to the load torque), as shown in FIG. 2, by a vector operation based on the output frequency and the phase of electric current for each phase with respect to the output voltage. Accordingly, the magnitude of the weight of load 18 can be determined with high accuracy from the torque current value by detecting a motor current and dividing the motor current into an exciting current and a torque current by a vector operation.
In this regard, the magnitude of the load weight can be determined substantially accurately from the torque current value in the case of an electrically-driven rope hoist as disclosed in Patent Literature 2, but load weight determination cannot be accurately performed in the case of the electric chain block for the following reason. The electric chain block winds up and unwinds, by a polygonal sheave 15, a chain 16 having vertical links 16a and horizontal links 16b of the same substantially oval configuration that are alternately joined to each other. With this structure, the load torque varies even for the same load weight, and the torque current value varies periodically, which makes it impossible to perform accurate load weight determination. As shown in FIG. 3, the position of the center line A, B of the chain 16, i.e. the load weight center, moves away from and toward the center of rotation of the sheave 15 within a predetermined range ΔL according to the angle of rotation of the sheave 15. In response to the movement of the load weight center away from and toward the center of rotation of the sheave 15, the load torque applied to the sheave 15 varies within a predetermined range. The variation of the load torque causes variation in the value of electric current supplied to the lifting-lowering motor 12 from the inverter controller 11. It should be noted that the reference symbol Lc in FIG. 3 denotes a length of the chain 16 corresponding to one link thereof.
FIG. 4 is a graph showing the change of torque current during a low-speed lifting operation of the electric chain block. Curve A shows the change of torque current for a rated load weight (load; 1.0 W), i.e. (current/rated current [%]), and curve B shows the change of torque current for a rated load weight×1.08 (1.08 W). FIG. 5 is a graph showing the change of torque current during a high-speed lifting operation of the electric chain block. Curve A shows the change of torque current for a rated load weight (1.0 W), i.e. (current/rated current [%]), and curve B shows the change of torque current for a rated load weight×1.08 (1.08 W). It is required for an electric hoisting machine to surely lift when the load weight is not in excess of the rated load weight and to automatically stop the lifting-lowering operation when the load weight exceeds 1.08 times the rated load weight. When the difference between load weights to be distinguished is small as stated above, the load variation due to the polygonal sheave of the electric chain block becomes larger than the load variation due to variation in load weight, so that it is impossible to distinguish between a load of 1.0 W and a load of 1.08 W. Even during a low-speed lifting operation, it is impossible to distinguish between a load of 1.0 W and a load of 1.08 W at the time of starting the operation.